Multiple site communication systems which comprise a plurality of repeaters and transceivers that are distributed throughout a large geographic region are well known. Many multi-site systems use same-frequency simulcast, i.e., the same communication channel (or carrier frequency) is used by multiple sites throughout the region to simultaneously relay communications to subscriber communication units that are located throughout the multi-site system. This is an efficient frequency reuse technique when the subscribers are routinely located throughout the multi-site system.
The use of comparators, and their associated voting algorithms, within simulcast communication systems is known. In general, a comparator, operably coupled to a plurality of base stations or satellite receivers located in geographically diverse areas, attempts to select or construct a favorable representation of an audio signal given multiple sources of the signal (e.g., the base stations). This is accomplished by comparing the signals received from the signal sources and selecting, from amongst all of the signal sources, portions of the signal having the best signal quality. The selected portions are then reassembled to produce a favorable voted signal frame. The voted signal frame can then be retransmitted by a base station, thereby increasing the probability of good reception (i.e., intelligibly decoded audio) at the signal destination (e.g., a mobile communication device). The signal selected as the best by the comparator is typically distributed therefrom to the transceiver sites for simultaneous re-transmission.
More recently, digital simulcast radio communication systems have been developed. In such systems, digital information is formatted into information frames. Each information frame is made up of a plurality of information packets that are transported through the system infrastructure.
All simulcasted information packets are processed through a comparator which receives inputs from multiple signal sources and selects an input signal source based on predetermined criteria of signal quality. The comparator then assigns a launch time (launch timestamp) to each information packet received from the selected input signal source, and transmits the information packet over an infrastructure link to at least one of a plurality of base stations, where the information packet is temporarily stored in a buffer. Error correcting information is added to the information packet, and at the assigned launch time the resulting data packet is transmitted by the base stations.
Simulcast systems which employ launch timestamps require that exact replicas of the timestamped information data packets be distributed from the timestamping device to each of the simulcast transmitters associated with a given radio channel. This distribution presently requires that the comparator make multiple copies of each timestamped data packet. This requires packet replication capability in the comparator achieved with additional hardware and/or software processing. This packet replication process makes impractical any attempt to configure the comparator to individually support multiple simulcast channels simultaneously. Because packet replication necessarily requires a comparator to source (generate) multiple copies, the aggregate bandwidth demands of the comparator site can become overwhelming, and in most cases it is impossible to efficiently support too many transmitters, or too support more than one channel simultaneously, using a single comparator.
In a conventional, dedicated single-simulcast-channel comparator implementation as shown in FIG. 1, a single comparator 5 is assigned to perform packet replication for a given channel. Comparator 5 comprises a separate hardware controller card 6 which provides a time division multiplexed (TDM) bus 9 which is used to connect a controller card (which creates the seed copy of the timestamped packet) to one or more of the transmitter wireline interface cards 7, 8. The wireline interface cards 7,8 each provide a link interface to two transmitters 10, 11, 12, 13 on the channel. The TDM bus 9 can be used to make a single copy for each interface card (by each card reading the same slot) and then each card makes a second copy via a software process to support the second transmitter interfaced to that card. Conventional comparators utilize independent serial data connections to each transmitter 10, 11, 12, 13 associated with the simulcast channel. One copy is sent over each of these serial data connections for delivery to each simulcast transmitter.
Thus, while the prior art suffices in making the necessary packet replications, such processing significantly reduces processing efficiency at the comparator site since packet replication requires additional hardware and software processing and requires the comparator to source each of the transmitters associated with a given channel with independent serial (or circuit) data connections. Also, the comparator processing scheme as currently employed prevents a single comparator from being utilized to support multiple simulcast channels simultaneously. As a result, in a multiple channel simulcast system architecture, there are currently an equal number of comparators required as there are radio communication channels to support information packet distribution.
Therefore, a need exists for a comparator device which can be coupled to an existing network infrastructure and which would make possible simulcast (timestamped) packet replication and distribution to associated transmitters on any given channel in a manner which removes the burden of packet replication from the comparator device itself.